Yes, AT&T is my carrier. The big reason for that is their international services. They have a much better offering than T-Mobile, Verizon, or Sprint when it comes to international service. Oddly, I stayed tethered for nearly an hour with no problem yesterday in the same spot as where I had these issues the other day. :/

The only thing I can thing of at this point is that I need to get my SIM card changed out if I'm not going to go back to my iPhone. From what I'm reading, AT&T provisions SIMs to allow access to the higher speed networks based on the device in which it is being used, so a micro SIM from an iPhone will never be able to fully utilize 42 Mbps HSPA+.